1. Field of the Invention
The invention relates to powder lacquers based on a mixture of epoxide compounds salts of benzene polycarboxylic acids and optionally polyesters with primary diamines as hardeners.
2. Discussion of the Background
In industry there is a great need for powder lacquers of all types. This is primarily due to the fact that with their use no solvents are needed and there are only very small losses during application. Powder lacquers are economical and ecologically sound.
Most known lacquers produce glossy surfaces. In recent times, there has been more interest in flat surfaces. Also, it is often more advantageous to produce flat surface for safety reasons.
The simplest principle for obtaining a flat surface is to add to the powdered lacquer smaller or greater amounts of fillers, depending on the extent of the desired flat effect. Fillers such as chalk, finely distributed silicon oxide, barium sulfate or incompatible additives such as waxes, cellulose derivatives may be used. These additives have the effect of decreasing the technical quality of the lacquers however.
In DE-PS 23 24 696 a procedure for the production of lacquers with flat surface is disclosed in which a special hardener in the salt of cyclic amides with certain polycarboxylic acids is use. Actually, because of its excellent technical properties, this is the only procedure which has been economically successful. In the meantime, the procedure has been repeatedly improved (see DE-OS 30 26 455, OS 30 26 456 and German patent application P 35 11 998.5 of Apr. 2, 1985 "Hardener for powdery lacquers on the basis of epoxy resins").
However, in spite of this, this procedure has some basic disadvantages. The required amidines are expensive and not always available in the quantities needed. It would be desirable to be able to use more available basic compounds, such as directly available liquid diamines in a suitable form as a hardener for epoxy resins. However, the salts of these amines with the normally used carboxylic acids are, contrary to the amidine salts, so stable that under the baking conditions usual in the powder lacquer industry, there is no sufficient hardening of the epoxy resins.
In JP-OS 82/61 020, additive products of dodecamethylene diamine with an aliphatic dicarboxylic acid and carbon dioxide are disclosed which are supposed to effect the hardening of epoxy resins already at temperatures between 90.degree. and 120.degree. C. Tests have shown that under these conditions lacquers with satisfactory properties are not obtained. In particular there is no flat effect (see comparison tests). Thus, one would be discouraged from using this system for epoxy lacquers.
DE-OS 33 28 134 describes powder lacquers based on mixtures of epoxy compounds, polycarboxylic acids and reaction products of triacetone diamines with epoxides, urea and isocyanates, which produce flat lacquers after hardening. The triacetone diamine derivatives permit a hardening at temperatures of above 160.degree. C. with acceptable processing times. A disadvantage of this system is that it is limited to certain triacetone diamine derivatives which must first be produced and isolated during a preliminary reaction.